An epoxy resin, particularly, a glycidyl ether epoxy resin prepared from bisphenol A and epichlorohydrin includes a great variety of types ranging from a liquid resin to a solid resin having a high molecular weight, and which is widely used in various fields.
A liquid resin having a low molecular weight, on the one hand, has a high reactivity in epoxy groups, and which is cured at normal temperatures with a polyamine or polyamide resin by utilizing the property and used as an adhesive, an FRP, or a flooring material. Furthermore, it can be cured by heating with a polybasic acid anhydride for molded products, especially in the electric industries.
On the other hand, an epoxy resin having a high molecular weight is a brittle solid resin having a melting point ranging from 60.degree. to 150.degree. C., and since the epoxy resin contains terminal epoxy groups and secondary hydroxyl groups, it is widely used in the field of coatings by utilizing the reactivity of hydroxyl groups.
For example, an epoxy ester obtained by esterification of the solid resin with an unsaturated fatty acid has been used as an air-drying coating or a baking coating in which a melamine resin is used as a cross-linking agent.
Furthermore, the epoxy ester resin has been used as a can-coating by being combined with a phenolic resin.
Still further, the solid epoxy resin has been used as epoxy-based powder coatings or cationic electrodeposition coatings by mixing with a blocked isocyanate.
As described hereinabove, although the epoxy resin is used in the various fields, it is rigid and brittle, and the hydroxyl groups are secondary. Accordingly, the reactivity with a cross-linking agent to be reacted to the hydroxyl groups is poor and a high temperature is required in baking and cross-linking. Furthermore, the epoxy resin has a disadvantage in that the weatherability is poor.
U.S. Pat. Nos. 4,522,984 and 4,521,570 [corresponding to the Japanese Patent Examined Publication (Kokoku) Nos. 1607/1987, 20251/1988, and 20252/1988] discloses an epoxy resin having an improved reactivity, a process in order to improve the poor reactivity in the secondary hydroxyl group, and a curable resin composition therefrom.
By Kokoku No. 1607/1987, the flexibility in the epoxy resin can be moderately improved by the ring-opening polymerization of epsilon-caprolactone and, further, the secondary hydroxyl group which is poor in reactivity can be changed to the primary hydroxyl group which is excellent in reactivity.
Kokoku No. 1607/1987 states that 97 to 5 parts by weight of an epoxy resin having secondary hydroxyl groups such as Araldite 6097, Araldite 6084, and Araldite 6071 which are products manufactured by Ciba-Geigy AG, is allowed to react with 3 to 95 parts by weight of epsilon-caprolactone in the presence of a catalyst such as a titanium compound, for example, tetrabutyltitanate or a halogenated tin compound at 100.degree. to 200.degree. C. to obtain an epsilon-caprolactone-modified epoxy resin having primary hydroxyl groups by ring-opening reaction.
However, even the epsilon-caprolactone-modified epoxy resin having primary hydroxyl groups has a problem that it is insufficient in water resistance.
In order to improve the insufficient water resistance in the epsilon-caprolactone-modified epoxy resin described hereinabove, an another attempt was made for modifying the secondary hydroxyl groups to primary hydroxyl groups.
As a result of an intensive investigation by the present inventors, it was found that an improved epoxy resin having a water resistance and further hydroxyalkyl carbonate groups which include primary hydroxyl groups having an excellent reactivity can be obtained by ring-opening reaction of a specified cyclic alkylene carbonate with an epoxy resin having secondary hydroxyl groups.
Furthermore, it was found that insufficient water resistance can also be improved without the loss of flexibility by the ring-opening reaction of a specified cyclic alkylene carbonate with the epsilon-caprolactone-modified epoxy resin described in the U.S. Patent.
In the meantime, a film-formable resin having hydroxyl groups can provide a cured article having three-dimensional network structures by using a curing agent such as a polyisocyanate compound or a melamine resin.
Accordingly, the resin has been used as a vehicle for curable coatings. Hitherto, acrylic resins, polyester resins, and epoxy resins, etc., have been used as the film-formable resin for purpose.
It is known that a film-formable resin having hydroxyl groups which are situated at a longer distance from the main structure in the polymer has a higher reactivity. Therefore, reactivity has been improved by the reaction of a lactone compound with hydroxyl groups in the film-formable resin. However, the resin modified by the lactone compound alone has the possibility of poor water resistance.
The thermosetting resin composition of the present invention has a higher reactivity than curable compositions prepared using conventional resins because the hydroxyl groups are situated at a greater distance from the main structure.
Furthermore, the thermosetting resin composition of the present invention has excellent water resistance because the main chains of the polymer and hydroxyl group are combined through carbonate groups.